The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving layer and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. The DTR-image can be formed in the image receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
Two main types of mono-sheet DTR materials that are distinct because of their different layer arrangement and processing are known. The first type of mono-sheet DTR material comprises on a support, generally paper or a resin support such as polyester, in the order given a silver halide emulsion layer and an image receiving layer containing physical development nuclei as a surface layer. After information-wise exposure and development according to the DTR process a silver image is formed in the surface layer. Since the underlying layers are hydrophilic in nature and the silver image formed on the surface is hydrophobic or can be rendered hydrophobic the thus obtained plate can be used without further processing. These type of printing plates have a low printing endurance typically around 10000 copies.
On the other hand mono-sheet DTR materials are known that comprise a hydrophilic support provided with an image receiving layer containing physical development nuclei and on top thereof a silver halide emulsion layer. After information-wise exposure and development according to the DTR-process a silver image is formed in the image receiving layer. In order to obtain a lithographic printing plate it will then be necessary to remove the now useless silver halide emulsion layer to uncover the silver image formed in the image receiving layer. Said removal is generally carried out by rinsing the element with cold or warm water. This type of printing plate is disclosed e.g. in EP-A-278766, EP-A-483415 and EP-A-410500.
As for other printing plates it is required that the printing plates obtained according to the above described DTR-process have a high printing endurance, good ink acceptance in the printing areas and no ink acceptance in the non-printing areas (no staining). With respect to these requirements it has been experimentally stated that that the complete removal of all of the gelatin of said emulsion layer (or any other gelatin containing layer) from the silver image formed in the image receiving layer is of the uttermost importance. Even by applying intermediate layers, facilitating the removal of said gelatin layer(s) as disclosed in EP-A-483415 and EP-A-410500, there remain spots of gelatin on said silver image formed in the rimage receiving layer especially when the removal of said gelatin layer(s) is made with rinsing water at room temperature, resulting in bad ink acceptance in the printing areas.